#19084: fix: Brave Search baseUrl and Inter-session Message Role

This PR addresses two issues:\n\n1. Adds 'baseUrl' support for the Brave Search provider, allowing users to route search requests through proxies or custom endpoints (#19075).\n2. Fixes an issue where inter-session messages (sent via sessions_send) were incorrectly attributed to the 'user' role instead of 'assistant' (#19049). <!-- greptile_comment --> <h3>Greptile Summary</h3> This PR adds `baseUrl` support for Brave Search to enable routing through proxies/custom endpoints, and fixes inter-session message role attribution. **Key changes:** - Added `braveBaseUrl` configuration option in `types.tools.ts` and properly threaded it through the web search implementation - Updated cache key generation to include `braveBaseUrl` to prevent cache collisions between different endpoints - Fixed inter-session messages (sent via `sessions_send`) to be correctly attributed to "assistant" role instead of "user" role The changes are well-structured and follow the existing patterns in the codebase. The Brave baseUrl implementation mirrors the existing Perplexity baseUrl support, and the message role fix correctly handles inter-session communication. <h3>Confidence Score: 4/5</h3> - This PR is safe to merge with low risk - The changes are straightforward and follow established patterns in the codebase. Both features (Brave baseUrl support and inter-session message role fix) are isolated, well-implemented, and address real issues. The cache key update prevents potential bugs from endpoint mixing. No security concerns or breaking changes identified. - No files require special attention _{Last reviewed commit: 171118d} <!-- greptile_other_comments_section --> _{(2/5) Greptile learns from your feedback when you react with thumbs up/down!} <!-- /greptile_comment -->